Microfluidic devices are used in a wide range of fields for precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter scale. Microfluidic structures include microsystems for the handling of off-chip fluids (liquid pumps, gas valves, etc.), as well as structures for the on-chip handling of nano- and picoliter volumes. To date, the most successful commercial application of microfluidics is the inkjet printhead. In inkjet printing, small droplets of ink are controllably directed toward a recording medium in order to form an image. Although the majority of the market for drop ejection devices is for the printing of inks, other markets are emerging such as ejection of polymers, conductive inks, or drug delivery. Advances in microfluidics technology are also utilized in recent molecular biology procedures for enzymatic analysis, DNA analysis, and proteomics. Microfluidic biochips integrate assay operations such as detection, as well as sample pre-treatment and sample preparation on one chip. Another emerging application area is biochips in clinical pathology, especially the immediate point-of-care diagnosis of diseases. In addition, microfluidics-based devices, capable of continuous sampling and real-time testing of air/water samples for biochemical toxins and other dangerous pathogens, can provide an always-on early warning.
Many microfluidic devices include a patterned polymer layer on a substrate, such as silicon, such that the patterned polymer layer includes walls for fluid passageways to direct the flow of fluid, or for chambers for constraining a small quantity of fluid. Typically the substrate includes one or more inorganic layers formed on a surface of the substrate, where the inorganic layers form structures for operating on the fluid in the microfluidic device in some fashion. The patterned polymer layer is typically formed over the inorganic layer(s). Adhesion of the patterned polymer layer to the inorganic layer(s) is important during fabrication as well as during storage and use of the microfluidic device, and it is well-known to apply an adhesion promoter on the inorganic layer(s) prior to applying the polymer material, or to incorporate adhesion promoter within the polymer material prior to applying it to the inorganic layers. Typical polymer layers are photo-sensitive polyimides and photo-sensitive epoxies. The family of photo-sensitive epoxies called SU-8 is prevalent in microfluidic devices, due to properties such as high stability to chemicals, excellent biocompatibility, and the ability to form high aspect ratio structures such as walls having a greater height than width.
Selection of an appropriate adhesion promoter is generally dependent upon the type of polymer layer that is used in the microfluidic device. The adhesion promoter provides bonding sites for the polymer material, as well as for the inorganic layer(s). A common class of adhesion promoter materials is the organofunctional alkoxysilane materials. The alkoxy groups are methoxy or ethoxy groups. These alkoxy groups can be displaced by hydroxyl groups in the inorganic layer(s), so that the surface of the inorganic layer(s) is silanized. In other words, covalent —Si—O—Si— bonds are formed at the surface.
Organofunctional alkoxysilane materials also include an organic function for promoting bonds to the polymer material. Organofunctional alkoxysilane materials are classified according to their organic functions. For example, in aminosilanes the organic function is a primary or secondary amine. Aminosilanes are conventionally used as adhesion promoters for promoting the adhesion of polyimide to silicon or other inorganic materials, since the amino group promotes adhesion to polyimide. A typical aminosilane adhesion promoter intended for improving the adhesion of polyimide is VM-652 (having an active ingredient of a-amino propyltriethoxysilane) available from HD Microsystems. For glycidosilanes the organic function is an epoxide. Glycidosilanes are conventionally used as adhesion promoters for promoting the adhesion of epoxies to silicon or other inorganic materials, since the epoxide group promotes adhesion to epoxies. A typical glycidosilane adhesion promoter intended for improving the adhesion of epoxy is A187 silane, or Z6040 (having an active ingredient of 3-glycidoxypropyltrimethoxysilane) available from Dow Corning. U.S. Pat. No. 6,409,316 describes the use of Z6040 as an adhesion promoter for SU-8 type epoxy for use in a thermal inkjet printing device.
Some fluids used in microfluidic devices weaken the adhesion at the interface between the patterned polymer layer and the inorganic layer(s). Such attack at the interface can be accelerated if the microfluidic device is stored or used at elevated temperature. Although the conventional glycidosilane adhesion promoters are found to work well to provide good adhesion for epoxy polymer layers to the inorganic layer(s) for the case of no exposure to fluids, or short-term exposure to fluids, or exposure to less aggressive fluids, it has been found that conventional glycidosilane adhesion promoters do not provide sufficient long-term adhesion for epoxy polymer layers exposed to some types of fluids, such as some aqueous based liquids.
What is needed is a microfluidic device and a method for making such a microfluidic device having improved adhesion of the epoxy polymer layer, particularly after extended exposure to fluids such as aqueous based fluids. An example of a microfluidic device intended for handling aqueous based fluids is an inkjet printhead used with aqueous based inks. Such inkjet printheads can include drop-on-demand printing devices from which drops are ejected as needed (e.g. by resistive heaters or piezoelectric actuators) in order to form an image. Inkjet printheads also include continuous inkjet printing devices where a continuous stream of liquid is forced through the device and formed into droplets which are selectively allowed to proceed to the recording medium or deflected to a gutter for recycling.